One of the reasons WordPress is the most popular blogging platform is its theme system. WordPress allows its users with almost unlimited design options when modifying or designing themes. This is a collection of 40 resources that every theme designer can appreciate.

Starting Points:

Blank Themes - If you’re creating your own WordPress themes, these blank themes are a great starting point. They’ll save you time by giving you the bare-bones structure and still allow you the creative freedom to design whatever you want.

Color Tools:

A great deal of your theme’s success will depend on the color choices you make. Here are some helpful tools for deciding on the right colors.

Color Schemer Gallery
COLOURlovers
Website Color Picker
I Like Your Colors

Codex Info:

If you have much experience at all with WordPress, you’ve probably at some point referred to their Codex (a source of information on all things WordPress). Here are some of the Codex articles that are most relevant to theme designers.

Theme Development
FAQ on Layout and Design
The WordPress Loop
Anatomy of a Template Tag
Search Engine Optimization for WordPress
Accessibility
Photoblogs and Galleries
Developer Documentation

Icons:

Almost every blog uses icons in one form or another. Here are 4 high-quality sets of icons that are great for your blogging needs.

RSS Icons - Most blog readers now recognize the common, orange RSS icon. Using the icon is a good way to draw attention to your subscription link.
Mini Icons from Brand Spanking New - A free set of greyscale icons that includes such icons as speech bubbles that can be used on blog themes.
Blog Icon Pack from graphicPUSH - Some icons that are commonly used on blogs.
Web 2.0 Icons from Utombox - A number of icons for popular web 2.0 sites like Technorati, Flickr, FeedBurner and more.

Articles and Tutorials:

Here are some of the best articles available for helping you to learn more about all aspects of WordPress themes.

Customize Your Own WordPress Theme
WordPress and SEO
Optimize WordPress for Search Engines
Blog Design 101: Creating Your Own WordPress Theme
Secrets of WP Theming - Part 1
How to Create a WordPress Template or Theme

Photos:

Pictures and images can make a huge difference for a theme. The resources below will provide free or low-cost images. Be sure to check the restrictions if you are distributing the theme. Many of the photos will be limited to your own use and prevent distribution. If you find a photo that you really like you can always contact the photographer and ask them for permission.

Stock.xchang
iStockphoto
Stockxpert
flickr Creative Commons

Theme Galleries and Places to Distribute:

All of these sites list themes that are available for download. It’s not easy to get a theme listed, but it can really help to get more exposure and downloads.
WordPress Theme Viewer
WPThemesFree.com
Weblog Tools Collection

CSS Galleries:

You’ve worked hard creating a beautiful design. CSS galleries are a great way to get some exposure for your designs.
CSS Vault
CSS Based
CSS Beauty

Support:

You’ll probably experience some difficulties along the way. Most of your questions are already answered in the forums and you can always post new ones.
WordPress Forums

Validators:

If you’re going to distribute your theme you’ll probably want to make sure it validates.
HTML
CSS
RSS/Atom

Browser Testing:

Your visitors will be using many different browsers, which requires testing on your part.
BrowserShots - Get screen shots of your site in a number of different browsers.
BrowserCam - A paid service for testing a site.